jfgordy: Just clarify a little more. The personal "a" is used when the direct object is a "known person" (or pet). For example: He is going to visit his girl friend = "Él va a visitar a su novia"////////He is going to visit a girl = "Él va visitar una chica".
kjfalco: Yes, that is a possible translation, but it appears Duolingo does not like this version. It is called "Present Progressive" and Duolingo will not accept it. Maybe some day they will if enough people complain.
Why isn't the personal a used when saying: (He visits the doctor) Él visita el doctor.
This is another example that showed up for me prior to this example, Duolingo's answer did not have a 'personal a' in the answer.